/Engine/Source/Runtime/Engine/Public/HighResScreenshot.h
Utility function for merging the mask buffer into the alpha channel of the supplied bitmap, if masking is enabled. Returns true if the mask was written, and false otherwise.
bool MergeMaskIntoAlpha ( [TArray](API\Runtime\Core\Containers\TArray)< [FColor](API\Runtime\Core\Math\FColor) > & InBitmap, const FIntRect & ViewRect )